This invention relates to removing contaminants from a surface. More particularly, the invention relates to the removal of contaminants from a substrate surface by irradiation that does not alter the molecular crystal structure of the treatment surfaces.
Surface contaminants include discrete pieces of matter that range in size from submicrons to granules visible to the unaided eye. Such contaminants may be fine dust or dirt particles or unwanted molecules comprised of elements such as carbon or oxygen. Contaminants frequently become adhered to a surface by weak covalent bonds, electrostatic forces, van der Waals forces, hydrogen bonding, coulombic forces or dipole-dipole interactions, making removal of the contaminants difficult.
In certain instances, the presence of surface contaminants renders the contaminated substrate less efficient or inoperable for the substrate's designated purpose. For example, in certain precise scientific measurement devices, accuracy is lost when optical lenses or mirrors in the devices become coated with microfine surface contaminants. Similarly in semiconductors, surface defects due to minor molecular contaminants often render semiconductor masks or chips worthless. Reducing the number of molecular surface defects in a quartz semiconductor mask by even a small amount can radically improve semiconductor chip production yields. Similarly, removing molecular surface contaminants, such as Carbon or oxygen, from the surface of silicon wafers before circuit layers are deposited on the wafer or between deposition of layers significantly improves the quality of the computer chip produced.
Moreover, a significant portion of the debris that ultimately contaminates silicon wafers during production emanates from production apparatus such as process chambers in which the wafers are placed and pipes that conduct processing gas to the chambers. Accordingly, the level of wafer contamination experienced during the course of production can be significantly reduced by the periodic cleaning of such apparatus.
The need for clean surfaces free of even the finest contaminants has led to the development of a variety of surface cleaning methods. These known methods, however, each have serious drawbacks. For example, wet chemical cleaning eliminates metal ions and soluble impurities, but fails to remove particulates. Conversely, scrubbing techniques eliminate particulate, but employ devices that require regular maintenance and risk damaging the treatment surface due to physical contact. Similarly, pressurized fluid jet cleaning facilitates removal of particulates but risks damaging treatment surfaces due to the high pressure at which the cleaning fluid is maintained. Further, this technique may electrostatically damage the treatment surface due to the presence of ions in the cleaning fluid. Ultrasonic cleaning is a technique that may also result in physical damage to the treatment surface due to the intensity of sound waves being conveyed in the liquid medium. Additionally, "Megasonics," a high pressure chemical delivery system, may contaminate treatment surfaces with the contents of the very chemical solutions intended to remove contaminant. Similarly, strippable polymer tape may also contaminate treatment surfaces by depositing a polymer residue thereon. Finally, like "Megasonics" and polymer tape, each of the foregoing cleaning techniques employ cleaning tools and/or agents that can introduce as many new contaminants to a treatment surface as they remove.
Another known method for cleaning substrate surfaces without outside agents requires that the treatment surface be melted to release contaminants which are then removed by ultra high vacuum. This method has the disadvantage that the surface being treated must be briefly melted. Such melting may be undesirable, as for example when a semiconductor surface is cleaned between deposition of circuit layers and it is desired that the integrity of the previously deposited layers not be disturbed. Further, such an operation would be difficult if not impossible to implement for cleaning expansive, irregular surfaces, such as those found in pipes and wafer processing chambers. Finally, the ultra high vacuum equipment used in this process is both expensive and time consuming to operate.
Annealing treatment methods suffer similar drawbacks. When a surface is cleaned by annealing methods, the treatment surface of the substrate being cleaned is heated to a temperature that is generally below the melting point of the material being treated but high enough to enable rearrangement of the material's molecular crystal structure. The surface being treated is held at this elevated temperature for an extended period during which time the surface molecular crystal structure is rearranged and contaminants are removed by ultra high vacuum. Annealing cleaning methods cannot be used where it is desired to maintain the molecular crystal structure of the substrate surfaces.
Another currently utilized cleaning method, known as ablation, suffers from its own particular drawbacks. With ablation, a surface or contaminants on a surface are heated to the point of vaporization. Depending on the material being ablated, the material may melt before being vaporized or the material may sublimate directly on heating. With ablation cleaning techniques, if damage to the treatment surface is to be prevented, the ablation energy must be applied accurately to the contaminants only, rather than the surface on which the contaminants lie, a difficult task when the contaminants are extremely small or randomly spaced, or when the surface being treated is irregularly shaped. Even where the ablation energy can be successfully applied only to the contaminant, it is difficult to vaporize the contaminant without also damaging the underlying treatment surface.
Surface cleaning by melting, annealing and ablation can be conducted with a laser energy source. However, using a laser energy source to remove contaminants from a surface by melting, annealing or ablation does not overcome the inherent disadvantages of these processes. For example, in U.S. Pat. No. 4,292,093, "Method Using Laser Irradiation For the Production of Atomically Clean Crystalline Silicon and Germanium Surfaces" the laser annealing method disclosed requires both vacuum conditions and energy levels sufficient to cause rearrangement and melting of the treatment surface. Other known laser surface cleaning methods involving melting or annealing require similar high energy lasing and/or vacuum conditions, as disclosed in U.S. Pat. Nos. 4,181,538 and 4,680,616. Similarly the laser ablation technique disclosed in U.S. Pat. No. 3,464,534, "Laser Eraser" suffers the same drawbacks as other high energy ablation methods.